Research identifies seed resistant to Asian soybean rust

While Asian soybean rust hasn't become the monster some feared, the specter of an ASR epidemic still looms over the United States. So far, the only thing to combat the disease has been fungicides.

While fungicides have worked well, Mid-South farmers need a source of resistance to ASR “very badly,” says Billy Moore, who has headed Mississippi's ASR team for several years. “Already, even without a huge ASR epidemic, a large number of fungicides are going out. A good, resistant variety would lessen the need for so many sprayings.”

Southern soybean farmers will be pleased to know research into varieties resistant to Asian soybean rust is showing positive results. With the help of Southern university researchers, Schillinger Seed recently announced the discovery of resistant lines.

“We've been chasing ASR resistance since it came on the scene (in the United States),” says Bill Rhodes, Schillinger vice president of research, from Puerto Rico, where he's currently checking lines.

“We've always been interested in having plenty of diversity in the things we're working on. That's why we have a lot of material — some of it sort of exotic — that we've been working with and trying to incorporate into some of the more adapted varieties. That began as protein increases or oil-quality traits.”

In 2005, a small set of lines containing some exotic material was sent to a University of Georgia nursery for appraisal. Later, Rhodes received a call from one of the researchers. “He said, ‘Hey, you've got some material here that looks pretty good against (ASR).’

“It was a lucky find. That's how this material came to light for us. We didn't initiate the research specifically for ASR, but we were fortunate to find there are genes tolerant to the rust.”

The resistant/tolerant lines have now been tested in Georgia for two years.

“They're excited about what they've seen in their nurseries. We've also had these things in our breeding nurseries for a couple of years. Right now, we've got some seed rows growing in Puerto Rico for seed increase. Then, we can get some yield trials running next season and see what type of agronomic characteristics are in these experimental lines.”

Following the Georgia discovery, Schillinger asked Moore and Malcolm Broome — both retired Mississippi State University researchers — to initiate a study of their own in Mississippi.

In July 2006, the Mississippi nursery was planted around Lucedale, Miss., and contained over 1,000 breeding lines derived from the original resistant lines. Indeed, many of the lines proved rust-resistant when ASR arrived in September.

In 2007, screening trials were established in Lucedale and Natchez, Miss. The rust arrived in Lucedale in late September and rust resistance was again expressed in 300-plus lines while susceptible varieties were severely infected by the rust organism.

Moore, working as a volunteer on the research, is enthusiastic about ASR resistant varieties.

“I hope what the Schillinger folks are finding in Puerto Rico is as impressive as what we found in George County,” says Moore. “It certainly looked to me as if they've got some good, high-yielding varieties for Mississippi. I was surprised at how good some of their varieties were and how the resistance to ASR was still holding up.

“Everyone should remember that we had a healthy ASR problem late last year. And even though deer got into the plots, there was enough green matter and plants that, when walking across rows doing the rating, you could find zero ASR and then, next row, there would be some of the worst ASR we've seen in the state. We rated the varieties from zero to 5 — all over the board. The ASR resistance in some of the varieties was outstanding.”

The latest tests showed ASR resistance held up just as it did in 2006. “Let me tell you, Malcolm and I were elated with that. We're still very excited about what we found there.”

With one exception, both Moore and Rhodes are unaware of anyone else having found ASR resistance in soybeans.

“The USDA has two genes for ASR resistance,” says Moore. “I understand they have some tests working around Quincy, Fla. They're trying to get multi-gene resistance in the same line.

“I'm not sure when USDA plans to release a resistant variety. But if everything goes as planned, I understand something from these resistant Schillinger lines should be available in 2011. They're speeding the process up.”

Moore doesn't deny there are questions about the long-term efficacy of the ASR resistance. “I hope the resistance doesn't break down. I'd be lying if I said we're not worried about that. However, it's held up perfectly well for the last two years.”

There are differences of opinion on ASR resistance. “Some believe that single-gene resistance won't last long — and I believe single-gene resistance is what Schillinger has. Others think it'll last a long time.”

The difference of opinion largely comes from South America, where single-gene resistance hasn't held up well. There, ASR is a constant threat. However, in the United States, ASR must be introduced into the soybean production regions anew every year.

“It's like starting a brand new ballgame every season. We have considerably lower inoculum potential in the United States than in South America. Our situation is very different.”

Even if ASR did blow through U.S. soybeans and resistance broke down, “there's a possibility that since ASR won't overwinter there, the ASR that returns the following year won't be able to overcome the variety's resistance again.

“It's strictly a guess, but I suspect a single-gene resistance should hold up for a handful of years, at least. That remains to be seen and my belief certainly isn't everyone's.”

Moore and Broome will conduct more tests on Schillinger lines in 2008.

Over the last three years, ASR has been a late problem for Mid-South beans. In 2006, by the time ASR began creating problems, Mississippi had already harvested its crop. In 2007, ASR came into the state late again — wheat-beans were hit and rust was easy to find in Bolivar County, Miss., and Washington County, Miss.

In 2008, Mississippi's first case of ASR came early. During the first week of January, ASR was found on kudzu running up the side of a building. In most of those situations, “another cold spell will take that kudzu out,” says Moore. “If it doesn't, we'll take care of the vine on the building.”

Despite being kept under control, ASR still worries Moore. In the Fort Adams area, there's been extremely heavy ASR. Some of the gullies there “are 40-feet deep, run a quarter mile, and are full of kudzu. Soon, we want to check the bottoms of those gullies to see if there's any living kudzu with overwintering rust.

“I've never seen so much rust as in that area (in 2007). When shown that kudzu, knowledgeable people said it's the worst incidence of ASR they'd seen outside South America. ASR was all over: hundreds and hundreds of pustules per leaf and the vine running as far as you could see.”

A few miles north of the gullies is a lake normally surrounded with soybeans. Last season, “we found rust at late R-4/R-5. The beans on the west side of the lake were sprayed. Interestingly, on the east side, rust never was a problem even though there was such heavy inoculum just a few miles south.”